A dipmeter is logging device which determines the dip of a formation intercepted by the borehole. Customarily, they use extendable arms which carry resistivity measuring pads on the arms for contact against the sidewall of the well borehole. The dipmeter is pulled up the borehole, measuring changes in formation resistivity (or its inverse conductivity) as it travels along the well borehole. The dip angle of the formation boundary or interface (a change in resistivity) is indicated by the incremental shift between the respective pads which note passage of the change in resistivity. It is well known to detect the boundary of a strata or formation, thereby determining the dip angle with respect to the borehole and an imaginary horizontal plane. Heretofore, current conductive electrode type dipmeters typically inject current into the formation in a low frequency range, typically about one to five kilohertz. Sharp angular dip resolution requires that the electrode buttons on the electrode pads be quite small. Excessive size degrades the sharpness of the measurement. In other words, vertical spatial resolution is poor if the buttons are large. Ordinarily, the dipmeter generates a relatively large current which is transmitted from some upper part of the sonde enclosing the dipmeter and which is returned to a lower part of the sonde. The current flow through the formation is dependent on resistivity. The formation current flow is intercepted by the small buttons on the pads. Ordinarily, all the pads extending from the dipmeter will measure the change in resistivity occurring at a boundary when the pads move or slide over the boundary intercept in the borehole.
The foregoing system works quite well so long as formation resistivity can be measured in isolation. Isolation, however, is not always possible. More wells are now being drilled with oil based muds. At the low kilohertz frequency range mentioned above, such muds are essentially nonconductive materials. Drilling muds form a mud cake on the sidewall of the borehole. If the mud cake is nonconductive, there is difficulty in making conductive contact by the small buttons mounted on the electrode pads of the dipmeter. Scrapers and other types of blades mounted adjacent to the electrodes have been used to cut the insulative mud cake so that better electrical contact can be obtained. That is noisy at best. At worst, it creates erratic signals which may be a result of noise so that the noise looks like boundary resistivity changes. This makes log interpretation much more difficult.
Such low frequency dependent dipmeters are additionally limited in highly conductive borehole fluids, particularly those which become commingled with brine. The brine is highly conductive. The current emitted by the electrodes on the dipmeter is transmitted in an altogether different fashion and the detected signals thus become much less reliable. To the extent that any measure of signal reliability is obtained by dragging contact between the buttons on the electrode pads and the adjacent formation, such signal improvement is overwhelmed by the increase in noise derived from dragging the button across the irregular surface of the rock formations. In U.S. Pat. Nos. 4,739,272 and 4,780,678 and also in a publication at the 28th Annual SPWLA Logging Symposium (1987) there is discussed an inductive dipmeter apparatus, which operates similar to miniaturized versions of conventional induction tools, and which contains both transmitter and receiver coil arrays in each pad. This method, while successful to a degree, suffers from the problem of requiring extraordinary accuracy in assembly of the coils to maintain mutual coupling balance, while the spatial resolution (in the range of one to two inches) is generally regarded as barely adequate for accurate dipmeter interpretation.
The present disclosure sets out a dipmeter which overcomes all these problems by providing an entirely different approach to dipmeter measurements. The present approach uses a high frequency induction measurement approach. Instead of having a point contact electrode, and relying on point contact with the formation to detect the boundary between adjacent strata, this approach utilizes small coils to detect signals from the formation. Moreover, a higher frequency is used, preferably in the range of about one to ten megahertz. The sonde is much simpler to construct because it does not have to be constructed to form an injected current which requires division of the sonde into electrically isolated components. In this particular instance, a transmitter coil forms a relatively high frequency field which is directed into the formation. The electrode mounting mechanism of this disclosure is thus the same as that used heretofore, i.e., multiple pads on mechanically linked arms are used. However, the mode of connection of the pads with the formation and particularly the mode in which the signal is obtained is markedly improved. Accordingly, an entirely different approach for obtaining dipmeter measurements is set forth. Even so, the mechanics of the dip meter tool remain substantially the same, and dipmeter log interpretation remain substantially the same.